Numerous sewing and oversewing operations are performed parallel to the edge of a piece of fabric. The fabric edge may comprise angles which require stopping of the sewing at a given distance from one of the sides of the angle, rotation of the workpiece being sewn, stitching again as far as the next angle, and so on.
Automation of this kind of stitching requires the capability of stopping the operation at a specific distance from the edge of the fabric and the capability of detecting the passage of this edge opposite a given point with respect to the needle. It is relatively simple to detect a clean edge of a tight opaque fabric. It is much more difficult to detect the same edge of a very transparent fabric and especially the edge of a second thickness of fabric to be sewn on a background, as in the case of pockets to be applied to clothes.
Numerous devices exist which are capable of detecting the edge of a piece of fabric when the edge appears. One such device employs a photoelectric cell situated opposite a transmitter. The accuracy of such devices is known to be relatively uncertain and their behavior varies enormously from one piece of fabric to another. It is also known to use similar apparatus emitting and receiving infrared radiation for detecting thickness variations and for defining a signal indicating the passage of an edge during an operation of applying one thickness on another. The accuracy of such an apparatus is low and requires a very long time to adapt the sensitivity of the apparatus to the nature of the fabric sewn, depending on whether the fabric is more or less opaque.
The present invention relates to an important improvement in the above described detectors of the second type by means of a device using signals received by the "cell" which makes it extremely simple to adapt the detector to different fabrics and which allows a sharp signal to be obtained from the device and which is simple to use in a logic circuit for driving a power device.